The purpose of this proposal is to investigate the regulation and function of cardiac cells in vitro. The techniques of laser microbeam irradiation will be combined with behavioral analysis, electron microscopy and electrophysiology to study problems of: contractility, induced arrhythmias, cellular pharmacology, regeneration and injury repair, and cell-cell interactions. Studies will be performed on ventricular cells from various age pre-natal and post-natal rats and pigs, auricular cells, and non-muscle cardiac endothelial and fibroblast cells. This is a renewal proposal of work funded for the last six years, and it focuses primarily on the use of electron microscopy and single cell electrophysiology to elucidate various cellular functions during and following exposure to a laser microbeam and various drugs. Whereas the major objective is to better understand the function of the cardiac cell in vitro, substantial work is also amied at describing the effects and elucidating the mechanisms of laser interaction with subcellular structures.